Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). Examples of tachyarrhythmias include supraventricular tachycardias (SVT's) such as sinus tachycardia, atrial tachycardia, and atrial fibrillation. The most dangerous tachyarrythmias, however, are ventricular tachycardia (VT) and ventricular fibrillation (VF). Ventricular rhythms occur when an excitatory focus in the ventricle usurps control of the heart rate from the sinoatrial node. The result is rapid and irregular contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram because they do not use the normal ventricular conduction system, the depolarization spreading instead from the excitatory focus directly into the myocardium. Ventricular tachycardia is characterized by distorted QRS complexes occurring at a rapid rate, while ventricular fibrillation is diagnosed when the ventricle depolarizes in a chaotic fashion with no recognizable QRS complexes. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death.
Cardioversion (an electrical shock delivered to the heart synchronously with the QRS complex) and defibrillation (an electrical shock delivered without synchronization to the QRS complex to terminate ventricular fibrillation) can be used to terminate most tachycardias, including SVT's, VT, and VF. The electric shock terminates the tachycardia by depolarizing all excitable myocardium which prolongs refractoriness, interrupts reentrant circuits, discharges excitatory foci. A class of cardiac rhythm management devices known as an implantable cardioverter/defibrillator (ICD) provides this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device.
Another type of electrical therapy for tachycardia is antitachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt reentrant circuits causing the tachycardia. Modern ICD's typically have ATP capability so that ATP therapy is delivered to the heart when a tachycardia is detected, while a shock pulse is delivered when fibrillation occurs. Although cardioversion/defibrillation will terminate tachycardia, it consumes a large amount of stored power from the battery and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. Generally, only cardioversion/defibrillation will terminate fibrillation and certain high rate tachycardias, while ATP can be used to treat lower rate tachycardias. An arrhythmia which is regarded as terminable by ATP therapy, based upon rate or other factors, will be referred to herein as a terminable arrhythmia.
In most ICD's with ATP capability, ventricular fibrillation (VF) is distinguished from ventricular tachycardia (VT) using rate-based criteria so that ATP or shock therapy can be delivered as appropriate. The heart rate is usually measured by detection of the time between successive R waves (i.e., ventricular depolarizations). A measured heart rate is classified as a tachycardia when the rate is in a VT zone, defined as a range of rates above a tachycardia detection rate (TDR) but below a fibrillation detection rate (FDR). A measured heart rate above the FDR, on the other hand, is in the VF zone and is classified as a fibrillation. In a typical device, a tachycardia with a heart rate in the VT zone is treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the pacing fails to terminate the arrhythmia. It is a primary objective of the present invention to provide a method and apparatus for delivering ATP therapy in a manner that increases the likelihood that ATP therapy will terminate an arrhythmia without resorting to a defibrillation shock.